The IrDA (Infrared Data Association) scheme is one of currently known free-space optical communication standards. The IrDA scheme uses IrLMP (Infrared Link Management Protocol) as the network layer protocol.
In IrLMP, the connection and disconnection with other devices is carried out primarily by station control which controls that connection and disconnection via an IrLAP (Infrared Link Access Protocol) which is a lower layer and a LSAP (Link Service Access Point) which logically controls the connection.
To connect to a device, as shown in FIG. 3, when the LSAP of the primary station (initiating station) receives, from the upper layer, a connection request command 1 (LM_con_req1) which contains data needed to establish a connection to the secondary station (responding station), the LSAP issues a connection request command 2 (LS_con_req2) to the station control of the primary station which then issues a connection request command 3 (LAP_con_req3) to the IrLAP (lower layer) of the primary station.
The IrLAP of the primary station receives the connection request command 3 and outputs an SNRM command at a transfer rate of 9,600 bps to the secondary station (responding device). The IrLAP of the secondary station, upon receiving the SNRM command, issues a connection request command reception notification 1 (LAP_con_ind1) to the station control which is part of the IrLMP layer of the secondary station. When receiving the notification 1, the station control of the secondary station issues a connection confirmation command 1 (LAP_con_rsp1) to the IrLAP of the secondary station.
The IrLAP of the secondary station receives the connection confirmation command 1 and outputs a UA response to the primary station at a transfer rate of 9,600 bps. The IrLAP of the primary station receives the UA response and communicates a connection confirmation command reception notification 1 (LAP_con_conf1) to the station control of the primary station. The station control of the primary station issues a connection confirmation command reception notification 2 (LS_con_conf2) to the LSAP of the primary station.
The LSAP of the primary station receives the connect command reception notification 2 and issues a data transmission request 1 (LAP_Data_req1) for a transmission of data present in the connection request command 1 to the IrLAP. Upon receiving the data transmission request 1, the IrLAP outputs an I frame to the secondary station. The transfer rate of the I frame in that situation is determined by the IrLAP layer (lower layer) by observing the exchange of the aforementioned SNRM command and UA response: for example, 115.2 kbps or 4 Mbps.
The IrLAP of the secondary station receives the I frame, issues a data reception notification 1 (LAP_Data_ind1) to the LSAP of the secondary station, and passes data to the LSAP of the secondary station.
The LSAP of the secondary station, upon the parameter reception, issues a connection request command 4 (LS_con_req4) to the station control of the secondary station. Upon receiving the connection request command 4, the station control of the secondary station issues a connection confirmation command reception notification 3 (LS_con_conf3) to the LSAP of the secondary station.
The LSAP of the secondary station issues, to the upper layer of the secondary station, a connection request command reception notification 2 (LM_con_ind2) which contains the data received from the primary station and needed to establish a connection. The upper layer of the secondary station receives the connection request command reception notification 2 and issues, to the LSAP of the secondary station, a connection confirmation command 4 (LM_con_rsp4) which contains data needed to establish a connection to the other device. The LSAP of the secondary station receives the connection confirmation command 4 and issues a data transmission request 2 (LAP_DATA_req2) to the IrLAP of the secondary station.
The IrLAP of the secondary station receives this and transmits an I frame to the primary station. Upon receiving the I frame, the IrLAP of the primary station issues a data reception notification 2 (LAP_Data_ind2) to the LSAP of the primary station. The LSAP issues connection confirmation command reception notification 4 (LM_con_conf4) which contains data received from the secondary station and needed to establish a connection. That ends the series of actions (connection sequence) by the IrLMP layer.
To disconnect from the other device, as shown in FIG. 4, the upper layer of the primary station issues a disconnection request command 1 (LM_disc_req1) which contains data needed for a disconnection. Upon receiving the disconnection request command 1, the LSAP of the primary station issues a data transmission request (LAP_Data_req) to transmit the data of the disconnection request command 1. The IrLAP of the primary station, upon receiving the data transmission request, transmits an I frame to the secondary station. The IrLAP of the secondary station receives the I frame and issues a data reception notification (LAP_Data_ind) to the LSAP.
The LSAP of the secondary station identifies the data in the data reception notification as indicating a request for a disconnection. The LSAP then issues a disconnection request 3 (LS_disc_req3) to the station control of the secondary station and issues, to the upper layer, a disconnection command reception notification (LM_disc_ind) containing data which came from the primary station and needed for a disconnection. Meanwhile, the LSAP of the primary station, after issuing a data transmission request, issues a disconnection request command 2 (LS_disc_req2) to the station control of the primary station.
The station control of the primary station receives the disconnection request command 2 and implements a disconnection, and issues a disconnection request command 4 (LAP_disc_req4) to the IrLAP of the primary station. The IrLAP of the primary station receives the disconnection request command 4 and issues a DISC command to the secondary station. The IrLAP of the secondary station receives the DISC command and transmits a UA response to the primary station. That ends the series of actions (disconnection sequence) by the IrLMP layer.    [Non-patent Document 1] Infrared Data Association Link Management Protocol Version 1.1 [Search conducted on Jan. 21, 2005], the Internet <URL: http://irda.affiniscape.com/displaycommon.cfm?an=1&subarticlenbr=7>
However, the conventional scheme requires the exchange of 6 packets (namely, the SNRM command, UA response, I frame, Ack, I frame, Ack) between the devices to make a connection and the exchange of 4 packets (I frame, Ack, DISC command, and UA response) to break up the connection.
In infrared communication, it takes at least a fixed period of time after an optical transmission for a transmission/reception module to be able to start reception for the following reasons. In communication between devices, the reception module is in operation even when a transmission is being made. The module thus receives light that is emitted by itself. In far-distance communication, the light emitted by the module is far more intense than the light coming from the other device. Capacitors and passive circuits in the receiving circuit must be discharged and reset to stop emission of light, so that the module can reliably receive light from the other device.
The waiting time for the switching is specified to 1 ms (milliseconds) or longer for data communication at a transfer rate of 4 Mbps. However, actual waiting times of about 10 ms occur. Summing this and a packet transmission time amounts to 100 ms to 200 ms for a LAP layer connection (exchange of an SNRM command and a UA response) and 50 ms to 100 ms for a LMP layer connection (exchange of an I frame and an Ack). Communication overhead, that is, the time between the start of a connection process and the start of a data transmission, gets longer. That causes a problem of low overall communication efficiency.
In addition, the IrDA scheme has another problem. If the initiating station does not receive a response from the responding station in a connection establishing process, the initiating station cannot establish the connection, still less establish a one-way connection or perform a one-way data transfer.